Solar radiation is the ultimate energy source for all eukaryotes on earth, and it also provides information about the time, season, and surrounding environments. How organisms respond to light and how photosensory receptors mediate light responses represent some of the fundamental questions in biology. Cryptochrome is a blue/UV-A light receptor and the only photoreceptor known to function in both plants and animals. The function of cryptochromes is not limited to the control of plant photomorphogenetic development; their functions in the circadian clock and photoperiodism have also been found to affect complex human health problems such as cancer, sleep disorders, and other behavioral disorders. Investigation of how cryptochromes regulate plant development would increase our understanding of a fundamental aspect of life, and the knowledge resulting from such studies is also relevant to the improvement of human health. We have recently discovered that blue light-induced cryptochrome phosphorylation play key roles in the function and regulation of cryptochromes and identified approximate 100 primary target genes of CRY2 action. We have also developed the new genetics screens to identify mutations affecting cryptochrome functions, including sec (suppressors of cry1cry2), ecc (enhancers of cry1cry2), and coc (CRY2-GFP overexpression counteractor). We have cloned 9 SCC genes, and one ECC gene. The objectives of this proposal are (1) to investigate the biochemical mechanism of cryptochrome phosphorylation; (2) to study how CRY2 affects expressions of its putative primary target genes in response to blue light, and (3) to continue identify and clone the SCC, ECC, and COC genes, and to investigate how the SCC genes that we have identified, especially SCC4-D (a putative RING E3) and SCC5-D (a putative kinase), may mediate cryptochrome signal transduction in Arabidopsis.